So the only way I could find to fix this is to edit the buildtools.
Open the buildtools .sln up in VS etc
You will find a folder called Configuration, look for BuildConfiguration.cs and add this to the bottom of it:
/// <summary>Enable exceptions for all modules.</summary>
[RequiresUniqueBuildEnvironment]
public bool bForceEnableExceptions = true;
then have a look for the file TargetRules.cs where you can find the existing property bForceEnableExceptions which will be set to false. Change that to true and rebuild the solution.
Close UE editor down and open it back up - and with a whole lot of luck, the dog should compile for you again.